Marine Records of the Southern Ocean

Today there are four main water masses south of the polar front: Antarctic Surface Water; Warm Deep Water (WDW); Circumpolar Deep Water (CDW); and Antarctic Bottom Water (AABW) (see Chapter 4 for further details). WDW flows into the Southern Ocean from the North Atlantic, Pacific and Indian Oceans. This nutrient-rich water mass upwells at the Antarctic Divergence, replacing sinking Antarctic Surface Water. CDW, the dominant water mass in the Southern Ocean, is a mixture of WDW and Antarctic waters. AABW forms as cold, saline shelf waters sink and mix with CDW prior to flowing northwards out of the Southern Ocean. The Antarctic coastal current flows from east to west around the continent. North of the Antarctic Divergence, prevailing westerlies drive surface waters towards the east, forming the ACC, which in places extends to the seafloor. In part, these oceanographic conditions are a response to climate on the Antarctic continent. For example, formation of sea ice increases the density of shelf waters, aiding the formation of AABW. Conversely, changing ocean-atmosphere circulation patterns influence global climate by determining the transport of heat and moisture onto the polar continent. A commonly cited example is the thermal isolation of Antarctica by the ACC.

The MMCT witnessed a major reorganization of global deepwater circulation patterns associated with the expansion of the Antarctic Ice Sheet (e.g. Woodruff and Savin, 1989; Wright et al., 1992; Flower and Kennett, 1995). The widespread presence of hiatuses in Southern Ocean sedimentary cores is one indicator of this Middle Miocene reorganization of ocean circulation patterns. Although these hiatuses present difficulties in reconstructing past oceanographic conditions using marine cores, a broad consensus regarding the history of some of the Southern Ocean water masses has emerged in recent years.

One proxy used to reconstruct ocean circulation patterns is the carbon isotopic composition of benthic foraminifera, as water masses generally become enriched in the light isotope of carbon as they age. Interbasinal offsets in benthic foraminiferal carbon isotopes indicate that the Early Miocene Southern Ocean received warm saline deep water sourced from low latitudes in the Tethyan region (Woodruff and Savin, 1989; Wright et al., 1992). The flow of this warm saline water mass likely ceased in the Middle Miocene, and the consequent reduction in meridional heat transport has been called upon as a trigger for the ice-sheet expansion in the Middle Miocene (Woodruff and Savin, 1989; Flower and Kennett, 1995).

The ACC connects the Pacific, Atlantic and Indian Oceans and thus plays an important role with respect to global heat transport. Inception of a deep circumpolar ACC possibly occurred near the Oligocene-Miocene boundary, well before the MMCT (e.g. Pfuhl and McCave, 2005; Lyle et al., 2007). However, a variety of different proxies all point to temporal variations in the intensity of the ACC since its inception. A reduction in surface-to-thermocline foraminiferal 818O gradients at DSDP Site 516 indicates reduced thermal stratification in the surface waters of the southwestern South Atlantic around 16 Ma (Pagani et al., 2000). At the same time, the abundance of alkenones at this site decreased, implying a reduced nutrient input to this area. These observations have been interpreted to reflect a reduction in Antarctic Intermediate Water caused by a reduction in the strength of the ACC (Pagani et al., 2000). Re-establishment of a strong surface-to-thermocline temperature gradient in this region points to renewed flow of the ACC coincident with the expansion of the Antarctic Ice Sheet in the Middle Miocene (Pagani et al., 2000).

Because the oceanic residence time of neodymium is comparable to the ocean mixing time, the neodymium isotope system can be used as a radiogenic isotope tracer of ocean water masses. A study of the neodymium isotopic composition of sediments from DSDP Site 266 in the Australian-Antarctic basin indicates an increased contribution of neodymium from the Kerguelen volcanic province during the Middle Miocene, suggesting enhancement of the ACC during the climate transition (Vlastelic et al., 2005).

Pacific ferromanganese crusts that grew in equatorial Pacific bottom water display a gradual change in their neodymium isotopic composition between

38 and ~20Ma, which has been interpreted as increasing flow of Southern Ocean waters into the Pacific Ocean via the Deep Western Boundary Current (DWBC) as a result of progressive build-up of the ACC (Van de Flierdt et al., 2004). The isotope trends in the ferromanaganese crusts reversed in the Middle Miocene, likely reflecting increased mixing in the Pacific as a result of increased production and export of AABW associated with the build-up of Antarctic ice (Van de Flierdt et al., 2004). Independent evidence for intensification of the DWBC associated with the Middle Miocene Antarctic Ice Sheet growth is found in the record of sortable silt mean size from ODP Site 1123 situated on the Chatham Rise (Hall et al., 2003). The sortable silt mean size increases with stronger near bottom current flow and selective deposition and winnowing. The ODP Site 1123 record indicates increased flow speeds during the MMCT, interpreted as reflecting increased export of Southern Ocean waters to the Pacific Ocean. Furthermore, the intensity of the DWBC was found to exhibit variability on the timescale of the 41 kyear orbital obliquity cycle (Hall et al., 2003).

High-resolution foraminiferal proxy records from the South Tasman Rise (southwest Pacific) give further independent evidence for a strengthening of the ACC associated with the ice-sheet expansion in the Middle Miocene. Shevenell et al. (2004) generated paired oxygen isotope (818O) and Mg/Ca records using a planktonic foraminifera, G. bulloides, from ODP Site 1171. The d18O record reflects a combination of temperature and the oxygen isotopic composition of seawater, the latter a function of global ice volume in addition to regional salinity. The Mg/Ca record is used as a salinity-independent temperature proxy. The ODP Site 1171 Mg/Ca record displays a ~7°C cooling (point-to-point) between 14.2 and 13.8 Ma, which suggests that the paired d18O record reflects a concomitant surface water freshening above the South Tasman Rise. These records are interpreted as reflecting an intensification of the ACC between 14.2 and 13.8 Ma in a series of steps paced by orbital cycles (Shevenell et al., 2004), although it has also been suggested that a component of the surface water freshening may represent massive melting pulses of the Antarctic Ice Sheet (Holbourn et al., 2005).

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